There are several ways to approach the problem of piston engine vibration. The most direct is to reduce it at its source, (i.e., make the engine "smoother operating"). This can be done in a number of ways, such as using multi-cylinder engines, using internal counter-balancing mechanisms, and so on. Unfortunately, all of these approaches tend to add cost to the engine. The simple, single cylinder piston engine remains one of the most cost effective prime movers available. Therefore, an innovative approach to the solution of this problem with regard to a single cylinder piston engine is a matter of some importance.
The second line of defense against engine vibration is the mounting system of the engine on the frame of the vehicle or machine to which it is attached. Again, the most cost effective way is to simply bolt the engine rigidly to the frame and to depend on the mass of the engine to reduce the amplitude of the engine vibration. In vehicles and machines in which the engine is a small part of the overall weight of the machine, this is often an effective solution. However, in lightweight machines (of which typical examples would be consumer/light duty commercial products such as garden tractors and small gensets), the engine comprises a substantial proportion of the overall weight of the machine. In these cases, rigidly fixing a single cylinder piston engine to the machine usually results in levels of vibration which are totally unacceptable to the operator or to the application.
For several years, the predominately accepted compromise to this problem in small lightweight machines, such as garden tractors, has been to use a two cylinder engine which is rigidly mounted to the frame or base of the machine. This has been attractive because of the relatively low cost of multi-cylinder gasoline engines. However, with the advent of small diesel powered commercial products, such as garden tractors, there is a greater need to use a single cylinder engine in an apparatus without an unacceptable level vibration.
Another approach to the problem of excessive vibration, is to use vibration isolating mounting systems. These systems typically consist of a spring element or an elastomeric connection which supports the weight of the engine and, to varying degrees, "isolates" the vibration of the engine. Those skilled in the art know that spring design is controlled by several factors, among them the weight of the engine and the frequency of vibration to be isolated. Typically, the weight of the engine and the vibration frequencies to be isolated are such that the use of a spring, with as low a spring rate as can be achieved, provides the optimum in vibration isolation.
Unfortunately, there are other effects. The secondary effects of using a low spring rate is that the spring mass of the system, comprised of the engine and it's mounting, will have a low natural frequency of vibration. This means that when the engine is exciting the system at that natural frequency, or some multiple of it or some frequency very near to it, the excitation force required to produce large excursions of the engine as supported in the mounting system will be very small. It also means that, when the engine is exciting the system at a low frequency in the range of the natural frequency of the mounts, isolations will be poor and a large amount of the engine's vibration will be conducted through the mounting system to the machine itself. In lightweight machines, such as garden tractors, a significant movement of the machine itself will result--usually to the extent of being objectionable to the operator.
There are two times during the operation of an engine that the engine will pass through that low natural frequency of the mounting system--start-up and shut-down. During those periods, the primary source of vibration energy will be the reaction torques which occur when the engine is coming up against cylinder compression and the engine is exchanging energy between the piston and its flywheel. As the piston comes up on its compression stroke, energy is removed from the flywheel; when the piston comes down on the expansion stroke, energy is returned to the flywheel. The result of this energy exchange is that the rotating system of the engine is alternatively speeding-up and slowing-down (i.e., accelerating an decelerating rotationally) and a reaction torque (i.e., of the engine against the mounting system) is produced. This is not only a significant design problem, not only from the standpoint of efficient packaging, but also from the standpoint of the perception of the equipment operator who may become alarmed by the engine's gyrations. For example, in a small machine, such as a garden tractor, the equipment operator would be jostled about. This is also a problem whenever the equipment operator has an operating station atop or in close contact with the engine. It is also a problem where noise enclosures or other packaging constraints make it necessary or desirable to limit the total excursion of the engine on it's mounting system. This is an especially important problem in view of the recognized energy savings associated with single cylindered diesel engines. Until the public can accept such engines as being safe, an easy to operate and control the energy savings of a diesel engine will be lost from the consumer market. Moreover, the polution associated with ordinary gasoline engines will continue to contaminate the atmosphere. Thus, a significant design problem remains to be solved.